This invention relates to a flat cathode ray tube display apparatus comprising a cathode ray tube having an envelope, a luminescent screen extending over a substantially flat faceplate, means in the envelope for producing and directing an electron beam substantially parallel to the faceplate, and a deflection electrode array within the envelope and operable by a drive circuit for deflecting the line scanning beam in a direction towards said faceplate in field scanning manner.
An example of such a flat cathode ray tube display apparatus is described in British patent specification No. 2,101,396B. In this example a line scanning beam is produced by an electron gun and electrostatic deflector arrangement and directed substantially parallel to the faceplate in a rear region of the tube before being turned through 180 degrees by a reversing lens at one end of the tube and introduced into a region between the deflection electrode array and the screen with the plane of the line scanning beam being substantially parallel to the faceplate. The electron beam is a low-energy beam, and in the particular example described is a low current, low voltage beam of around 400 volts acceleration. A channel electron multiplier is situated parallel to, and spaced from, the screen and the electron beam is deflected by the deflection electrode array over an input side of the electron multiplier to provide a raster scanned input thereto. Having undergone current multiplication within the electron multiplier, the beam is accelerated onto the screen by means of a high voltage field established between the output side of the multiplier and a backing electrode on the screen to produce a raster-scanned display picture. Such a display apparatus may be used for television or other video display purposes.
In the known apparatus, the beam is deflected progressively downwards over the input side of the multiplier in field scan fashion by selective energisation of a plurality of vertically-spaced horizontally elongate electrodes which form a deflection electrode array situated parallel to the faceplate on the opposite side of the electron beam path and which in conjunction with an electrode at a fixed potential over the input side of the multiplier, create deflection fields for the beam.
The electrode array is driven to achieve continuous vertical scan by applying ramp voltages to adjacent pairs of electrodes in turn successively, the timing of the ramp voltages to the electrodes of each pair being predetermined. This form of driving allows a small number of electrodes to be used in the array, typically around fifteen. However, because of the way in which the electrodes are driven, the drive circuit necessary is complicated. The drive circuit is provided externally of the tube's envelope and is interconnected with the electrodes of the deflection array via respective lines thereby requiring a large number of lead-throughs to be provided in the envelope. By driving two electrodes of the array at a time with suitably timed linear ramps approximately uniform spot-height and vertical linearity are obtained. However, the residual, spatially-periodic, variations in line-pitch can be noticeable, because they produce a corresponding variation in apparent brightness, and careful control of the ramp shape and start and stop times is necessary. Acceptable results with monochrome displays can be obtained.
British patent specification No. 2,181,319A describes a version of this known type of display apparatus for displaying full colour pictures. The described apparatus has a luminescent screen which consists of a repeating pattern of three phosphor elements adapted to luminesce in different colours respectively, and further includes colour selection electrodes disposed intermediate the output side of the electron multiplier and the screen which are operable to deflect the electron beam exiting from the channels of the multiplier and by appropriate control of which the beam can be directed selectively onto each of the plurality of phosphor elements.
Briefly the colour selection electrodes used in this apparatus are in the form of a pair of electrodes for each channel of the electron multiplier arranged on opposite sides of the channel axis by means of which the electron beam exiting from the channel can be deflected to one side or the other so as to impinge upon respective ones of the phosphor elements to display selectively first and second colours, e.g. red and blue. In the undeflected state, that is with no potentials or the same potentials applied to the pairs of electrodes, the electron beam is directed onto the third phosphor element of the repeating pattern to produce a green display.
This display apparatus enables, therefore, a colour picture to be produced using a single electron beam which is scanned in raster fashion over the input side of the electron multiplier, the required line and field scan deflectors operating on the beam prior to reaching the electron multiplier.
The display apparatus may be used in order to display television pictures according to a conventional standard scanning format, for example the PAL standard of 625 lines, 50 Hz field format where the input red, green and blue signals are derived from an RGB source such as a camera, telecine or from a PAL decoder.
There is described in the aforementioned British patent specification No. 2,181,319A, a driving technique for producing colour television pictures from such a tube in which line scanning and colour selection are performed sequentially at three times the normal rate, i.e. at 46.875 kHz for the
system, the red, green and blue components of each television line being stored, time-compressed and displayed in sequence during one standard line period of 64 microseconds. Thus each normal television line is presented as three individual, and respectively coloured, lines each of which is drawn in one third of the standard line period. This driving technique will hereafter be referred to as triple line sequential operation.
In one possible mode of operation, vertical field scanning is effected in a continuous manner. For this, approximately linear ramps are successively applied to successive adjacent pairs of the electrodes of the array in predetermined relationship giving a conventional raster. The number of the individual electrodes in the array, typically fifteen, is a compromise dictated by the need to achieve vertically-uniform spot-height and picture geometry and brightness, whilst keeping the number of tube lead-throughs and external circuitry needed to drive the electrodes to a minimum. This requires the ramps to be non-linear in a particular manner and their start and end times to be accurately defined. Depending on the selection sequence adopted, that is, the order in which the individual colour lines are drawn, it can be expected that with such continuous vertical scan visible colour line structure, crawl or flicker impairments to the display will occur.
In an alternative mode of operation, vertical scan is effected in a stepped manner with the three individual colour lines ideally being superimposed on one another and thereafter the scanning beam is deflected one step to the next television line position and the process repeated. This mode of operation obviates the aforementioned display impairments as the red, green and blue component of each television display line are overlaid and the eye cannot detect the residual temporal errors. This technique also makes it possible to ensure a uniform line pitch and raster brightness. Stepped deflection can be achieved by replacing the linear ramps by non-linear staircase waveforms derived from values stored in digital memories, each step taking place during line blanking. In order to ensure an acceptably uniform field scan by this method it is considered that the memories would need to have around 12-bit resolution and the subsequent analogue circuits would need to have a very high stability. Such components would likely add significantly to the cost of the display system.